Bioenergetics

Photosynthesis:
In the presence of UV light, a very special reaction takes place in plant cells containing chlorophyll (in chloroplasts). Photosynthesis is the reason that there is life on Earth. The reaction is:
Carbon dioxide + Water → Glucose + Oxygen
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
You can experiment on plants to see proof of photosynthesis. If you cover a section of a leaf in aluminium foil and leave it in the sun. That part of the leaf that has no UV light getting to it, will not be able to photosynthesise. If that leaf it treated (ethanol to remove the waxy cuticle) and covered in iodine water. The iodine turns black in the presence of starch. Areas where photosynthesis has taken place turn black. We do not test for glucose as the glucose produced is quickly converted and stored as insoluble starch.

Rate of photosynthesis:
The rate of photosynthesis is of particular interest when we look at growing food. In order to maximise the rate of plant growth, we need to maximise the rate of photosynthesis. In specialised greenhouses, the atmosphere is rich in carbon dioxide, the plants have plenty of water (the two reactants in the equation), the temperature is optimal and there is as much sunlight as there can be.
We can test the importance of light intensity during one of our required practicals

Limiting factors of photosynthesis:
When trying to maximise the rate of photosynthesis, particularly commercially, it is important to understand that there are limits:
• Temperature - just thinking that hotter environment = faster reaction is not entirely true, after a certain temperature, the enzymes in plants become denatured over 50°C.
• Carbon dioxide concentration - over a certain point, the plant cannot process it as fast as you provide it so it is wasted.
• Light intensity - no matter how much you add, if every chloroplast is working to its full capacity, no amount of extra light will increase the rate.

Glucose and its importance:
We have discussed the importance of oxygen, however, from the point of view of plants and the point of view of the earliest cyanobacteria from around 3.5 billion years ago, oxygen is a waste product. The thing that plants want is the glucose. Glucose is used by us in respiration (the reverse of photosynthesis equation), it is converted to starch (God bless potatoes), plants also use it to make oils, fats, and the very fabric of the plants themselves - cellulose. Glucose is essential for making amino acids which are used to make proteins. As we "are what we eat", photosynthesis made the raw ingredients that we are made from. In fact, as we are carbon-based life forms, all of the carbon in us was taken from the atmosphere by photosynthesis and we ate it.
More importantly is the idea of oils and starch coming from photosynthesis which means that we have plants to thank for chips and crisps!

Aerobic respiration:
Aerobic respiration is happening in all cells when there is a plentiful supply of oxygen. There is a misconception that only photosynthesis occurs in plants, plant cells undergo respiration too. The equation for aerobic respiration is:
Glucose + Oxygen → Carbon dioxide + Water
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
As well as these two products, energy is also transferred from the chemical store into the thermal store. This is how mammals keep warm. This is the reverse of the equation for photosynthesis.

Anaerobic respiration:
If respiration is taking place and there is not enough oxygen, it is anaerobic. Because of the lack of oxygen, lactic acid is produced and not as much energy is transferred. This lactic acid remains in your muscles and can cause cramps. After you have finished your exercise, you will need to pay off the oxygen debt in which this extra oxygen reacts with the lactic acid and it breaks down to carbon dioxide and water.
Although not a good thing in human muscles, anaerobic respiration with yeast in solution of sugars is called fermentation. This is the carbon neutral method of making ethanol (alcohol) for use in alcoholic drinks, solvents and hand sanitiser.
Glucose → Ethanol + Carbon dioxide
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

Metabolism:
Metabolism is the term used for all of the reactions happening in an organism. These include:
• Making lipids and fats.
• Making amino acids and turning these into proteins.
• Respiration.
• Converting glucose into insoluble starch for storage.
These reactions (and their rates) go from glucose and nitrates making amino acids then proteins and finally being broken down to waste products like urea which is excreted in urine. This can be absorbed by plants from the soil (if you had a tinkle nearby) as a source of nitrogen.